This chapter describes the interactions of microorganisms with dairy foods that lead to commonly encountered product defects. The major microbial inhibitors in raw milk are lactoferrin and the lactoperoxidase system. Fluid milk, cheese, and cultured milks are the major dairy products susceptible to spoilage by non-spore-forming fermentative bacteria. Non-spore-forming bacteria responsible for fermentative spoilage of dairy products are mostly in either the lactic acid-producing or coliform group. The most common fermentative defect in fluid milk products is souring caused by the growth of lactic acid bacteria. The defect in noncultured fluid milk products is usually caused by growth of specific strains of lactococci. Spore-forming bacteria that spoil dairy products usually originate in the raw milk. The defect in milk products is described as sweet curdling, since it first appears as coagulation without significant acid or off flavor being formed. The major heat-resistant species in milk is Geobacillus stearothermophilus (formerly Bacillus stearothermophilus). A practical means to prevent sporeformers from spoiling nonfermented liquid dairy products given sub-ultrahigh-temperature (UHT) heat treatments has not been developed. The most common yeasts present in dairy products are Kluyveromyces marxianus and Debaryomyces hansenii (the teleomorph) and their asporogenous counterparts (the anamorph), Candida species, and Zygosaccharomyces microellipsoides. Yeasts and molds that spoil dairy products can usually be isolated in the processing plant on packaging equipment, in the air, in salt brine, on manufacturing equipment, and in the general environment (floors, walls, ventilation ducts, etc.).